1. Technical Field
The present disclosure relates to a volume holographic element, a volume holographic element manufacturing method, and a display device.
2. Related Art
In a volume holographic element, a configuration, in which a first substrate 5A, a holographic material layer 4, an adhesion layer 6 (intermediate layer), and a second substrate 5B are sequentially laminated, has been proposed (refer to JP-A-7-234627), as illustrated in FIG. 11. When a volume holographic element having the above-described configuration is manufactured, interference exposure is performed in such a way that the holographic material layer 4 in an uncured state or in a semi-cured state is irradiated with object light L1 and reference light L2, which are acquired by dividing light flux emitted from a common light source, parts which have different refractive indexes are formed in the shape of stripes in the holographic material layer 4 in the interference exposure process, and then the holographic material layer 4 is cured in a curing process.
However, in the interference exposure process, some of the exposure light is reflected in the interface between the first surface 5A0 of a first substrate 5A, which is a surface opposite to the holographic material layer 4, and an air layer, or in the interface between the second surface 5B0 of a second substrate 5B, which is a surface opposite to the holographic material layer 4, and an air layer, and thus interference fringes are generated in locations other than a predetermined location. Therefore, there is a problem in that it is difficult to appropriately form stripes having different refractive indexes (diffraction grid) in the holographic material layer 4. For example, when the reference light L2 is irradiated from the side of the second substrate 5B in the oblique direction, some of the reference light L2 is obliquely reflected in the interface between the first surface 5A0 of the first substrate 5A and the air layer, and thus the interference fringes are generated in locations other than the predetermined location of the holographic material layer 4, as expressed by arrow L20. Meanwhile, the object light L1, which is vertically irradiated from the side of the first substrate 5A, is vertically reflected in the interface between the second surface 5B0 of the second substrate 5B and the air layer. However, when the reflection direction of the reflected light is obliquely inclined, it causes the interference fringes to be generated in locations other than the predetermined location.